A nonaqueous electrolyte secondary battery, particularly a lithium secondary battery, has been put into practical use and is being widely used as a small power source for cellular phones, notebook computers and the like. Also, demands for a nonaqueous electrolyte secondary battery as a large power source for electric vehicles, distributed power storages and the like are on the rise.
However, lithium used in the lithium secondary battery is not an abundant resource, and future depletion of lithium resources is feared. On the other hand, sodium belonging to the same alkali metal is present abundantly as a recourse compared with lithium and one digit lower in the price than lithium. Furthermore, sodium has a relatively high standard potential, and therefore it is expected that the sodium secondary battery can be a high-capacity secondary battery. Examples of the sodium secondary battery include a secondary battery using a sodium-containing positive electrode active material for the positive electrode and using metallic sodium or sodium alloy for the negative electrode; and a secondary battery using a sodium-containing positive electrode active material for the positive electrode, and using a carbonaceous material or the like for the negative electrode. If a sodium secondary battery can be used in place of the existing lithium secondary battery, this enables mass production of a large secondary battery such as in-vehicle secondary battery and secondary battery for distributed power storages, without fear of resource depletion.
Meanwhile, as a positive electrode active material used for the positive electrode of a sodium secondary battery, it is disclosed, for example in Japanese National Patent Publication No. 2004-533706, that raw materials are mixed and calcined at 750° C. for 8 hours and obtaining sodium iron phosphate (NaFePO4) thereby, and the sodium iron phosphate is used for the positive electrode active material.